The present invention relates to a method and a device for the correction of signal distortions in an amplifier device, and in particular to a method and a device for the correction of signal distortions in a driver circuit of a class-D power amplifier.
A pulse-width-modulated signal (PWM signal) has its signal information in the pulse width and not in the amplitude. Therefore, generally switches, such as for example relays, switching transistors, thyristors or triacs, are used for the amplification of a PWM signal. The switching devices thereby switch a load alternately between two potentials, generally of a supply voltage and a reference potential, such as for example ground, the-time duration of the switched-on and switched-off state being determined by means of the pulse width.
However, switches of this type do not have an ideal switching behavior, i.e. errors occur, caused by the switching itself, such as for example switching delays, rise times and fall times (timing error). In addition, the supply voltage represents a further source of errors (power supply error), since interference voltages on the supply voltage have a direct effect on the output signals of the amplifier device. In addition, the load switched by means of the switching device or amplifier device itself disturbs the supply voltage.
In class-D power amplifiers, a pulse-width-modulated (PWM) signal is raised in the level of its voltage and/or current, i.e. amplified, in order to produce a signal for the direct or prefiltered driving of a load with a low impedance, such as e.g. a loudspeaker in audio applications. Although the input PWM signal can be produced digitally with very high quality, as far as distortions and/or the signal-to-noise ratio are concerned, the output stage has limitations in this respect. It is neither possible to produce a perfectly digitally switching driver stage nor is it possible to provide a power supply for the driver stage without distortions in the output voltage.
In practice this means that a correction circuit is required for correcting the errors which are caused by the power stage.
In WO 98/44626 a description is given of a feedback method which adapts the pulse width of a PWM signal directly and continuously over time, and thereby reduces the errors of the driver stage. A controlled variable thereby directly controls a correction unit in which the pulse width is changed as a linear function. No allowance is made there for the fact that the influence of the controlled variable on the output signal is dependent on the pulse width of the PWM signal at the time, i.e. a constant timing correction for small pulse widths brings about a considerably greater effect than for greater pulse widths. In practice this means that the controller must constantly readjust the pulse width, to be precise in addition to the error that is actually to be corrected. Such constant readjustment can in turn lead to interference signals and is consequently to be reduced or avoided as far as possible.
In the conference publication No. 393 by S. Logan, M. O. J. Hawksford “Linearization of Class D Output Stages for High Performance Audio Power Amplifiers” of the conference on Advanced A-D and D-A Compression Techniques and Their Application, Jul. 6 to 8, 1994, a description is given of a method in which the pulse width can be readjusted by a delay line with discrete taps. The control itself also takes place in this case by means of a feedback loop. The main difference in comparison with WO 98/44626 is, however, that in WO 98/44626 the pulse width is set continuously and in the conference article it is set in discrete stages.
WO 00/46919 describes a method in which the errors of the amplifier bridge are corrected digitally, i.e. before the actual PWM modulator. A disadvantage of this method is restricted accuracy, i.e. the smallest readjustment stage is determined by the resolution of the PWM modulator. In addition, there is the disadvantage that an A-D converter is required for digitizing the error.
In WO 99/45641 a description is given of a method in which the pulse width difference between an input signal and an output signal of the output stage is used as a controlled variable for correction. A disadvantage of this method is attributable to the fact that only timing errors of the driver circuit can be picked up in this way. According to the publication, controlling of the pulse width takes place by means of changing the amplitude of a triangular reference signal.